Properties of CTLs

If they encounter (on a dendritic cell) the antigen/MHC for which their TCR is specific, they

enter the cell cycle and go through several rounds of mitosis ("clonal expansion") followed by

differentiation into effector ("killer") cells. Their differentiation includes forming a large number of modified lysosomes stuffed with proteins: perforin and several types of granzyme. [see below] They are helped in these activities by helper T cells that secrete stimulatory cytokines like IL-21.

Most of these CTLs will die (of apoptosis) when they have done their job, but some (especially those that have received "help" from helper T cells) will become memory cells — long-lived cells poised to respond to the antigen if it should reappear.

An example will show the beauty and biological efficiency of CTLs.

Every time you get a virus infection, say influenza (flu), the virus invades certain cells of your body (in this case cells of the respiratory passages). Once inside, the virus subverts the metabolism of the cell to make more virus. This involves synthesizing a number of different macromolecules encoded by the viral genome.

In due course, these are assembled into a fresh crop of virus particles that leave the cell (often killing it in the process) and spread to new target cells.

Except while in transit from their old homes to their new, the viruses work inside of your cells safe from any antibodies that might be present in blood, lymph, and secretions.

But early in the process, infected cells display fragments of the viral proteins in their surface class I molecules. CTLs specific for that antigen will be able to bind to the infected cell and often will be able to destroy it before it can release a fresh crop of viruses.

In general, the role of the CD8+ T cells is to monitor all the cells of the body, ready to destroy any that express foreign antigen fragments in their class I molecules.

Some CD4+ T cells can develop into CTLs, but they can attack only those cell types (e.g. B cells, macrophages, dendritic cells) that express class II MHC molecules. Virtually every cell in the body expresses class I MHC molecules, so CD8+ CTLs are not limited in the targets they can attack.